A number of continuous wattage autotransformers (CWAs) and peak lead autotransformers (PIAs) have been used as ballasts for gas discharge lamps such as high pressure sodium (HPS) lamps and metal halide (MH) lamps. These types of ballasts, however, do not exhibit the overall superior performance desired for some lighting applications such as the illumination of large industrial facilities and roadways where fixtures are elevated and/or spaced far apart and are otherwise difficult for service personnel to reach for maintenance purposes. Accordingly, a number of customers desire a better quality ballast, such as an electromagnetic regulator, for these types of applications. Although electromagnetic regulators can be twice as expensive as CWA/PLA-type ballasts, a number of customers are willing to incur the additional expense to use electromagnetic regulators in lieu of CWA/PLA-type ballasts for lighting applications requiring superior ballast performance.
An electromagnetic regulator transfers energy from a primary winding connected to a power source to a tertiary or capacitive winding. The electromagnetic regulator then transfers energy in the tertiary winding to a secondary winding connected to a lighting fixture. Unlike other ballasts, the windings in an electromagnetic regulator are isolated. The tertiary winding regulates the volt-amperes in the regulator and can be manipulated to change flux levels in the core of the regulator. Thus, the tertiary or capacitance winding processes energy in the core, whereas the capacitance in other types of ballasts represent merely a lamp current limiting impedance.
Electromagnetic regulator circuits for discharge lamps are disclosed in U.S. Pat. Nos. 4,891,562 and 5,216,333, to Nuckolls et al, which are both hereby incorporated herein by reference in their entirety. The circuit disclosed in U.S. Pat. No. 4,891,562 uses an electromagnetic regulator having primary, secondary and tertiary windings connected to an alternating current (AC) power source, a lamp and a control circuit, respectively. The control circuit comprises an inductive reactor, a timer, a reference signal circuit, a comparator circuit, a direct current supply circuit and a semiconductor switch. The firing angle of the switch is controlled to regulate the operating characteristics of the lamp to compensate for line voltage changes, lamp aging effects and other variations. While the circuit is desirable for a number of reasons, the circuit uses a considerable number of components and is therefore costly.
The circuit disclosed in U.S. Pat. No. 5,216,333 is depicted in FIG. 1. The circuit is a step-dimming apparatus for a discharge lamp 22 having an electromagnetic regulator 10. The electromagnetic regulator 10 comprises primary, secondary and tertiary windings 12, 14 and 16, respectively, coupled together by a laminated core 18. The primary winding 12 is connected to an alternating current (AC) power source. The secondary winding 14 is connected to the discharge lamp 22 via a starting circuit 21. The tertiary winding 16 is connected to a capacitor 24 and is the capacitance winding of the regulator 10.
With further reference to FIG. 1, the tertiary winding 16 is provided with a center tap 26 for operating a reactor 30. The reactor 30 is connected in series with a normally closed contact set 32 in a switchable conductive path extending between one end of the winding 16 and the tap 26, that is, across one-half of the winding 16. When an actuator such as relay 28 opens the contact set 32, the reactor 30 has no effect on the lamp. When the contact set 32 is closed, the reactor 30 extracts positive volt-amperes and stores energy each half-cycle, thereby reducing the amount of energy stored in the capacitor 24. Accordingly, the energy available to the lamp is reduced and the lamp is dimmed.
The dimming circuit disclosed in U.S. Pat. No. 5,216,333, although desirable for a number of reasons, can also be too costly and too large for some applications, particularly due to the use of the reactor 30. A need exists for a dimming circuit having an electromagnetic regulator which avoids the need for an additional magnetic device such as a reactor, yet reduces the operating wattage level of a discharge lamp without creating lamp instability or power supply line problems.